Three campaign officials familiar with the process told Politico, a political news website, that Harris' campaign aides spent a lot of time and effort investigating a candidate's background, with the goal of uncovering a nominee's possible "dark history" in advance.

Because the F.B.I. does not conduct background checks on vice presidential candidates, Eric Holder, who served as attorney general in the Obama administration, and his law firm have taken on the task of selecting and screening candidates. Holder, the first African-American attorney general in U.S. history, is seen as Obama's most trusted adviser.

Each candidate must answer more than 200 questions before being placed on the list. At the same time, censors scrutinize a candidate's tax returns, financial records, relationship history and medical history. Candidates are required to submit accounts and passwords for their private social media accounts. Today, the social platform accounts of a nominee's children and even grandchildren are also subject to intense scrutiny.